WASHINGTON  Japanese manufacturers continue to make the cleanest-burning vehicles, but automakers are generally doing a poor job in lowering emissions that contribute to global warming, an environmental group said Tuesday.

Honda Motor Co. got the highest rank in the Union of Concerned Scientists' biennial report, which focused on 2003 vehicles from the six largest automakers in the U.S. market in terms of sales.

Honda vehicles produced less than half the pollution of the industry average, the report said. General Motors Corp. ranked lowest, with a fleet that produced a third more pollution than average.

Nissan Motor Co. ranked second and Toyota Motor Corp. was third, followed by Ford Motor Co. and DaimlerChrysler AG. Nissan and DaimlerChrysler moved up from their positions in the group's 2002 report.

The group said Nissan had the largest improvement in reducing the greenhouse gas emissions that many scientists, including the Concerned Scientists, believe are causing the Earth to become warmer. Nissan registered the second-largest improvement in reducing smog. DaimlerChrysler showed a modest improvement in the fuel economy of its trucks, which make up a substantial portion of its sales.

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The six automakers produce more than 90 percent of emissions in the United States, the group said. Researchers measured performance on two types of emissions -- smog-forming pollution and carbon dioxide, the main greenhouse gas linked to global warming -- and ranked automakers according to an average of the two.

David Friedman, research director of the Union of Concerned Scientists' Clean Vehicles Program and an author of the report, said federal and state anti-smog regulations have forced automakers to improve smog emissions. But carbon dioxide emissions haven't shown much improvement.

"In terms of heat-trapping gas emissions, automakers have been running in place for almost the last 20 years," Friedman said. "The industry is more or less ignoring global warming when you look at the products they're putting on the road."

Friedman said Honda has been a leader in reducing carbon dioxide emissions but is falling behind as its truck sales increase without improved technology.

GM moved from the best of the Big Three in the group's 2000 report to the worst. GM was the only automaker whose vehicles emitted more smog and carbon dioxide emissions in the 2003 model year than two years earlier, the report said.

GM spokeswoman Joanne Krell said the report doesn't take into account the mix of vehicles each company offers. For example, Honda has no trucks that compare in size to GM's largest models, like the huge Hummer H2, with a curb weight of more that three tons.

Friedman responded that even when compared to other companies with large fleets, GM is behind.

"It's not about what you sell. It's what you put into those trucks," Friedman said. "DaimlerChrysler sells more trucks than GM, but beat them out because they make cleaner trucks."

Krell also said the Environmental Protection Agency has rated some GM vehicles highly for fuel efficiency, including the Chevrolet Malibu Maxx, which is the best in its class in 2005. The company also delivered a fleet of hybrid buses to Seattle earlier this year.

"We're pleased that we are noted for having some very fuel-efficient products," Krell said.

The Union of Concerned Scientists, an environmental group formed nearly 40 years ago, said it would like to see automakers devote more of their marketing efforts to green vehicles and support federal and state environmental laws.